On July 26, 2016, a strange, slow, silent aircraft landed at Abu Dhabi International Airport, completing a journey that had begun 16 months earlier at the same airfield. Solar Impulse 2 had just circumnavigated the Earth — crossing Asia, the Pacific Ocean, North America, and the Atlantic — powered entirely by the sun. It carried no fuel. Its four propellers were driven by electricity generated by 17,248 solar cells spread across wings wider than a Boeing 747's. It had spent 23 days, 14 hours, and 29 minutes in the air across 17 legs and five continents.

The pilots, Swiss explorers Bertrand Piccard and André Borschberg, alternated legs. On the longest single stretch — Hawaii to California — Borschberg flew alone for 117 hours and 52 minutes, nearly five days, over the Pacific Ocean at an average speed of 47 miles per hour. He slept in 20-minute naps, monitored by a ground team that woke him if his brain waves indicated he was falling into deep sleep. The flight landed him in the Guinness World Records for the longest solo flight in history, measured by both time and distance.

A Vision Twenty Years in the Making

Bertrand Piccard came to Solar Impulse from a family tradition of pushing the limits of what machines can do. His grandfather, Auguste Piccard, reached the stratosphere in a pressurised gondola in 1931. His father, Jacques Piccard, descended to the deepest point in the ocean — 10,916 metres into the Mariana Trench — in 1960. Bertrand himself completed the first non-stop around-the-world balloon flight in 1999, in the Breitling Orbiter 3, arriving in Egypt after 19 days, 21 hours, and 47 minutes.

The around-the-world balloon flight made him think differently about fuel. "When we landed," Piccard said, "we had eight hundred kilograms of propane left. It was the fear of running out of fuel that almost paralysed us during the whole flight." He began asking what it would take to fly with no fuel at all — to be carried by the energy of the sun alone. It took twelve years and $170 million to find out.

The Engineering of Nothing

Solar Impulse 2 was an exercise in extreme lightness. With a wingspan wider than a 747 and a weight equivalent to a large car, it had to be built from materials that barely existed in commercial quantities when design began. The carbon fibre airframe weighed 2.3 tonnes. The 17,248 monocrystalline silicon solar cells — spread across the wings, fuselage, and horizontal stabiliser — charged four lithium polymer battery packs during the day, storing enough energy to fly through the night at reduced power. The aircraft could fly indefinitely, in principle, as long as the sun kept shining and nothing broke.

The cockpit was unpressurised and barely large enough for the pilot to recline slightly. For overnight legs and oceanic crossings, the pilot wore a heated suit and used supplemental oxygen at altitude. The 20-minute nap schedule was developed with sleep researchers: short enough to prevent the deep sleep that causes grogginess, frequent enough to provide adequate rest across days-long flights. Every major aviation record for solar-powered flight was broken multiple times during the programme.

Por qué es importante

Solar Impulse 2 was not a practical aircraft. At 47 mph average speed, it would take six weeks to fly from London to New York. Its passenger capacity was one. Its payload was essentially nothing. It was, in operational terms, impractical — which was, in some ways, the point. The flights were not meant to demonstrate a product. They were meant to demonstrate a principle: that renewable technology, applied with sufficient ingenuity, can do things that seem impossible.

In the century and a bit since Kitty Hawk, aviation has consumed approximately 35 billion tonnes of aviation fuel. The atmosphere has paid for every one of those flights in carbon. Solar Impulse 2's circumnavigation was a single data point in a much larger argument — about what flight might look like in the next hundred years, if the next generation of engineers is willing to make things that are as radically different from today's aircraft as the Wright Flyer was from Abbas ibn Firnas's silk-and-feather wings. The sky hasn't changed. Only our ability to navigate it has — and that ability is still evolving.

Sources: Bertrand Piccard and André Borschberg, Solar Impulse project; Wikipedia, "Solar Impulse"; The Guardian, July 26, 2016